In recent years, a high-temperature superconducting (HTS) magnet around which HTS wires are wound have a broader range of applications. For example, rotary machines such as a superconducting rotary machine using this HTS magnet have been developed.
By the way, the critical temperatures of HTS materials are about 90K. For this reason, it is necessary to cool windings of the HTS wires to about several K to several tens K by use of a cooling device or the like. In particular, in a superconducting rotary machine for a marine vessel (ship), a great thermal load is generated. To secure a sufficient cooling capability, for example, a cooling device such as a GM freezing (refrigerating) device is used. In a case where the superconducting rotary machine for the marine vessel which uses the HTS material employs a rotation field type (revolving field type) in which field elements including HTS field poles rotate, a direct cooling method cannot be used, in which the cooling device is directly connected to the HTS magnet and the field poles are cooled by conductive heat transfer. Therefore, the cooling device and the rotor are thermally connected to each other by utilizing a cooling medium (refrigerant) reflux. Such a method is implemented by, for example, a thermosiphon cooling system which cools the HTS field poles by natural convection by use of a cooling medium (e.g., neon, nitrogen or the like) having been cooled by the cooling device such as the GM freezing device. For example, Patent Literature 1 discloses a superconducting machine as the superconducting rotary machine using the thermosiphon cooling system.